let p=new Promise((resolve,reject)=>{
    console.log('p1')
    setTimeout(resolve,1000)
})
p.then(()=>new Promise((resolve,reject)=>{
    console.log('p2')
    setTimeout(resolve,1000)
})).then(()=>new Promise((resolve,reject)=>{
    console.log('p3')
    setTimeout(resolve,1000)
}))